(a) Field of the Invention
The present invention relates to a toner for use in electrostatic photography, and more specifically relates to a pressure sensitive adhesive toner for use in electrostatic photography which is capable of readily being fixed to a surface by applying pressure to the toner.
(b) Background of the Invention
Heat fixable toners have been mainly used in electrostatic photography. Heat fixable toners are mainly composed of a thermoplastic resin, and they are prepared by the steps comprising softening said resin by heating, admixing a coloring agent therewith and then grinding same. In use, these toners are melted by the use of heating means such as heating rolls, infrared radiation or the like, and then they are fixed to form a visible image. However, the conventional fixation methods using thermofixable toners have involved the various problems enumerated hereunder;
(1) A long period of time is required for raising the temperature within the fixation apparatus to a temperature higher than the softening temperature of the toner used,
(2) A large amount of electric power is required for maintaining the temperature within the fixation apparatus higher than the softening temperature of the toner used,
(3) The use of an excessively high temperature within the fixation apparatus in order to increase the copying speed is liable to cause scorched copies to be produced and in an extreme situation, to cause a fire.
Proposals have been made to increase the copying speed while maintaining the temperature within the fixation apparatus at a low level by using toners which have a low softening point or which can be fixed merely by applying pressure thereto. For instance, Japanese Pat. Publication No. 9880/1969 discloses a pressure fixable toner composition comprising C.sub.6 -C.sub.25 aliphatic components, and Japanese Pat. Open Application Nos. 75033/1973, 78931/1973 and 78936/1973 disclose developers for use in electrostatic photography comprising soft polymers, respectively. However, preparation of these developers by means of conventional grinding methods is accompanied by a higher cost of preparation on account of (1) the necessity of cooling and grinding said composition or soft polymers at temperatures under their brittle points from the viewpoint of grindability and (2) the necessity of classifying the thus prepared toners, which have a wide particle size distribution, according to their particle sizes. Moreover, these toners are disadvantageous in the following points, (3) cohesion, bridging and caking take place during the preparation or storage of the toners, (4) adhesion occurs between the carrier particles and the toners, ( 5) the urface of the electrostatic latent image becomes coated with a film stain, which causes trouble in repetitive use.
Additionally, Japanese Pat. Publication No. 8561/1959 and U.S. Pat. No. 3,080,318 disclose pressure recording of encapsulated oily coloring agents. U.S. Pat. No. 3,080,250, U.S. Pat. No. 3,386,822 and Japanese Pat. Open Application No. 71648/1973 disclose methods of imparting fixability of toners by breaking solvent-containing capsules and so forth. However, the aforesaid prior art is not free from drawbacks such as, because the encapsulated core materials are liquid the images formed by using these toners are liable to be blurred and to exhibit flowing and to possess the foul odor of the solvents. Still further, Japanese Pat. Open Application No. 75032/1973 and Japanese Patent Open Application No. 17739/1974 disclose soft polymer-encapsulated pressure fixable toners. In these cases, large quantities of organic solvents are also used in the preparation of said toners, resulting in an increased cost owing to the recovery of solvents and so forth, the necessity of selecting non-combustible solvents from the viewpoint of fire prevention and the inevitable restriction in selecting the resins to be used. These toners, which are inferior in particle fluidity, are admixed with hydrophobic silica, etc., as a flow agent. Since these additives impede the toner's fixability, there is required a high pressure of 300-400 lb/in for the purpose of fixing the toner powder images. The prior toners are not fully satisfactory from the practical point of view.